While watts describes electrical power in a dynamic sense, current (amperage) describes electrical force in an absolute sense. Watts and current as electrical concepts are vaguely similar to horsepower and torque in mechanical engineering. Strictly speaking, watts (total power) is simply voltage (pressure) x amps (force). Horsepower is the amount of torque (force) generated over a specific time period (torque x time);(1 horsepower = 550 ft. pounds of torque x 1 second). Like torque, current represents the level of electrical "force" without reference to a dynamic element. A simple analogy that is easy to visualize is that amperage is the width of a water stream flowing from a hose with voltage representing the pressure of the water flowing from it. The total water dispensed over a given time would be watts. A fire hose can potentially deliver far more force than a waterpik using the same amount of water per second (although the waterpik would conceivably cut steel at such pressures - much like a high voltage arc might). Just as water hoses are rated for maximum pressures, so are electrical insulators rated for maximum voltages (electrical pressures) and not amps. Conductor size, like the diameter of a water hose, determines maximum current. When you exceed a conductor's current capacity, you don't electrically defeat the insulation but the conductor overheats (and sometimes melts). This is how fuses work. Current is a factor in the mechanical control of a loudspeaker element. Current, like torque, has its value in the ability to overcome the inertial resistance of a load - sometimes referred to as "reactive power". All loudspeakers are in some form or another a movable mechanical element attached to a electrical "motor". Some driver designs have greater inertial values - a greater mechanical load - and require larger amounts of current (reactive power) to properly control their oscillations. This is reflected by the measured impedence specifications of the driver at various frequencies (expect a 4 ohm speaker to draw more current than an 8 ohm unit). The moving elements of low frequency drivers (woofers) tend to have much more mass than high frequency elements (tweeters) - and generate more inertial forces as they occiliate. The "motors" (voice coils) of woofers consequently need greater current (reactive force) to keep the bigger driver (load) from oscillating out of control. Imagine waving in your arms a sheet of paper vs. a sheet of 4x8 plywood and you'll understand the difference. This is why reproducing low frequencies uses so much more total power (watts) - because typically more current (reactive force) is required at any given voltage. Maximum amperes are typically specified for better power amplifiers. Controlled high amperage is typically more difficult to achieve with tube amps because vacuum tubes can only pass low currents at very high voltages, which then have to pass through a transformer (a device which lowers volts and raises amps, but which is not as electronically "responsive" - because it has its own form of electrical resistance called "inductance" - as a straight wire). This is why SS amps tend to exhibit crisper bass response than typical tube designs. I hope this wasn't too long or complicated, but amperage, watts and voltage are intimately related and to appreciate one you should understand all.
